This invention relates to a receiver of a code-division multiple access system and, in particular, to a receiver of a code-division multiple access system intended to improve a reception quality.
In a conventional mobile communication system such as a mobile telephone, use has been made of a multiplexing system such as a time division multiple access (TDMA) system or a frequency division multiple access (FDMA) system. However, in response to a growing demand for effective use of frequencies following an increase in number of subscribers and for multimedia communications, attention is directed to a code division multiple access (hereinafter abbreviated to CDMA) system as a multiplexing system for a next-generation mobile telephone. The CDMA system is a technique for simultaneously carrying out a plurality of communications by the use of signals in a same frequency band by means of the spread spectrum technique. In a CDMA mobile communication system using the above-mentioned technique, a plurality of users occupy a same frequency and a same time and modulate communication data by the use of spread codes assigned to the users to identify the users. The spread codes of the users are orthogonal to one another. Therefore, at a receiving side a multiplexed signal obtained by multiplexing all user""s communication data is multiplied by a spread code used by each user in a same phase so as to extract communication data of a desired user.
In the CDMA mobile communication system, a communication quality is determined by orthogonality of communication data signals of all users multiplexed In the same frequency. Practically, however, due to variation in a propagation condition, the orthogonality can not completely be maintained. Therefore, when the signal of the desired user is demodulated a signal component of another user is undesiredly contained to result in deterioration in signal quality. In order to avoid the deterioration in signal quality, the receiving side measures a ratio between a signal reception level and an interference reception level for the desired user and requests a transmitting side to change transmission power so as to satisfy a predetermined ratio. In this approach, a transmission level is increased at the transmitting side in order to maintain a predetermined signal-to-interference ratio (hereinafter abbreviated to SIR) at a CDMA receiver in the CDMA mobile communication system. However, increase in transmission level prevents the reduction in power consumption at a terminal and the improvement in degree of multiplexing into the same frequency. In order to solve the above-mentioned problem, attention is directed to an interference removing technique. In the interference removing technique, an interference wave, i.e., a signal component other than that of a desired user is removed from a communication data signal received. Thus, it is possible to improve a reception signal quality even in a low SIR condition.
Hereinafter, description will be made of a CDMA receiver using the interference removing technique. Herein, it is assumed that the CDMA receiver performs an interference removing operation of a multistage type in which interference removal is repeatedly carried out in three stages for three users.
FIG. 1 shows the structure of a conventional CDMA receiver for carrying out interference removal in a multistage fashion. The CDMA receiver comprises a reception timing detecting section 10 for detecting reception timings of three users and, in correspondence to the reception timings, interference estimating sections in each stage. The interference estimating sections includes first- through third-stage interference estimating sections 1111 through 1113 corresponding to the reception timing of a first user, first through third-stage interference estimating sections 1121 through 1123 corresponding to the reception timing of a second user, and first- through third-stage interference estimating sections 1131 to 1133 corresponding to the reception timing of a third user. The CDMA receiver further comprises residual signal producing sections 121 and 122.
A multiplexed signal 13 received by the CDMA receiver is supplied to the reception timing detecting section 10, the first-stage interference estimating sections 1111 through 1131, and the residual signal producing section 121. The multiplexed signal 13 is a frame signal composed of a plurality of slots. At a predetermined position in the frame, a pilot symbol as predetermined pattern data is added before or after an information symbol of a predetermined length. The reception timing detecting section 10 detects the pilot symbol added to the multiplexed signal 13 to detect data reception timings of desired users. The reception timings thus detected are supplied as reception timings 141 through 143 to the first-stage interference estimating sections 1111 through 1131, the second-stage interference estimating sections 1112 through 1132, and the third-stage interference estimating sections 1113 through 1133 individually for the users, i.e., individually for the reception timings. In synchronism with the reception timings 141 through 143 detected by the reception timing detecting section 10 for the individual users, the first-stage interference estimating sections 1111 through 1131 multiply the multiplexed signal 13 by spread codes assigned to the individual users to extract data signals of the desired users, respectively. The data signals thus extracted are supplied as user signals 151 through 153 to the second-stage interference estimating sections 1112 through 1132 in a subsequent stage, respectively. In addition, the first-stage interference estimating sections 1111 through 1131 multiply the extracted user data signals again by the spread codes assigned to the users. Thus, signal components of the users contained in the multiplexed signal 13 are reproduced to obtain reproduction signals 161 through 163 which are supplied to the residual signal producing section 121. The residual signal producing section 121 is supplied with the multiplexed signal 13 in addition to the reproduction signals 161 through 163 and produces a residual signal 17 obtained by subtracting the reproduction signals 161 through 163from the multiplexed signal 13. The residual signal 17 is used as an input signal to be subjected to interference removal in the second stage.
The residual signal 17 is supplied to the second-stage interference estimating sections 1112 through 1132 and the residual signal producing section 122.In synchronism with the reception timings 141 through 143 detected by the reception timing detecting section 10 for the individual users, the second-stage interference estimating sections 1112 through 1132 multiply the residual signal 17 supplied thereto by the spread codes individually assigned to the users to despread the residual signal. Resultant signals (or despread signals) are weak in signal level. Therefore, in order to minimize errors produced in transmission-path estimation required upon demodulation, the user signals 151 through 153 supplied from the first-stage interference estimating sections 1111 through 1131 are added to the resultant signals to produce added user signals increased in ratio of the signal components of the desired users. Thus, data signals of the desired users are extracted. The data signals thus extracted are supplied as user signals 181 through 183 to the third-stage interference estimating sections 1113 through 1133 in a subsequent stage, respectively. In addition, the second-stage interference estimating sections 1112 through 1132 subtract, from the user data signals extracted thereat as demodulation signals, signal components corresponding to the user signals 151 through 153 previously added and multiply results of extraction again by the spread codes assigned to the users, respectively. Thus, signal components of the relevant users contained in the residual signal 17 are reproduced as reproduction signals 191 through 193 which are supplied to the residual signal producing section 122. The residual signal producing section 122 is supplied with the residual signal 17 in addition to the reproduction signals 191 through 193 and produces a residual signal 20 obtained by subtracting the reproduction signals 191 through 193 from the residual signal 17. The residual signal 20 is used as an input signal to be subjected to interference removal in the third stage.
Likewise, the third-stage interference estimating sections 1113 through 1133 extract desired user signals for the residual signal 20 and produce demodulation signals 211 through 213 of the desired users corresponding to the user signals 161 through 163 and 181 through 183 produced by the first- and the second-stage interference estimating sections 1111 through 1131 and 1112 through 1132, respectively. In this event, the residual signal 20 approaches nearer to zero than the residual signal 17 so that the third-stage interference estimating sections 1113 through 1133 produce the demodulation signals 211 through 213 from the added user signals after the interference is removed at maximum, respectively.
The above-mentioned technique related to the CDMA receiver is disclosed, for example, in Japanese Unexamined Patent Publication (JP-A) No. H10-190494 xe2x80x9cINTERFEERENCE CANCELLER AND CHANNEL ESTIMATIONxe2x80x9d.
However, in the conventional CDMA receiver already proposed, interference is not removed from the multiplexed signal itself supplied to the reception timing detecting section. Therefore, the reception timings of the desired users are detected from the reception signal containing interference waves at a great ratio. As a consequence, it is difficult to detect accurate reception timings. Furthermore, since the interference of the reception signal is removed with reference to such inaccurate reception timings, the reception quality is deteriorated to cancel the effect of interference removal.
It is therefore an object of this invention to provide a CDMA receiver which enables detection of accurate reception timings even in a condition that an SIR is low.
A CDMA receiver to which this invention is applicable is for receiving, as a reception signal, a signal given by subjecting a data signal comprising predetermined pattern data to spread modulation by the use of a spread code.
According to an aspect of this invention, the receiver comprises:
correlation value data producing means for producing correlation value data obtained by multiplying the reception signal by the spread code and the predetermined pattern data;
signal-to-interference ratio calculating means for calculating a signal-to-interference ratio of said reception signal; and
reception timing determining means for determining a reception timing of said predetermined pattern data in response to said correlation value data and said signal-to-interference ratio.
Preferably, the reception timing determining means determines the reception timing such that the maximum value of the correlation value data exceeds a predetermined first threshold value and that the signal-to-interference ratio exceeds a predetermined second threshold value when the correlation value data have the maximum value.
The signal-to-interference ratio calculating means may calculate the signal-to-interference ratio from the reception signal and the correlation value data produced by the correlation value data producing means.
According to another aspect of this invention, the receiver comprises:
correlation value data producing means for producing, at each sampling point within a predetermined time range, correlation value data obtained by multiplying the reception signal by the spread code and the predetermined pattern data;
correlation value data memorizing means for memorizing, in correspondence to the above-mentioned each sampling point, the correlation value data produced by the correlation value data producing means;
signal-to-interference ratio calculating means for calculating a signal-to-interference ratio of the reception signal;
signal-to-interference ratio memorizing means for producing an interpolating signal-to-interference ratio for the signal-to-interference ratio calculated by the signal-to-interference ratio calculating means for each sampling point within the time range based on a reception timing at which the signal-to-interference ratio is calculated and for memorizing the interpolating signal-to-interference ratios in correspondence to the above-mentioned each sampling point;
retrieving means for retrieving maximum correlation value data among the correlation value data memorized in the correlation value data memorizing means;
correlation value data judging means for judging whether or not the maximum correlation value data retrieved by the retrieving means exceed a predetermined first threshold value;
ratio judging means for judging, when the correlation value data judging means judges that the maximum correlation value data exceed the first threshold value, whether or not a particular signal-to-interference ratio memorized in the signal-to-interference ratio memorizing means in correspondence to a particular sampling point of the maximum correlation value data exceeds a predetermined second threshold value; and
reception timing determining means for determining, when the ratio judging means judges that the particular signal-to-interference ratio exceeds the second threshold value, a reception timing corresponding to the particular sampling point as a reception timing of the predetermined pattern data.